If you live in a big city and can pick from a bunch of disciplines, I'd say either muay thai or K-1 kickboxing for striking and either wrestling (folkstyle or greco-roman), judo or BJJ for grappling

If the only gyms in your area are regular ol' boxing gyms that's still a very good way to train, same goes for kyokushin karate specifically (can't recommend the other types of karate for learning self-defense)